You misunderstand. I'm not holding down the mouse button, moving, or copying files. I just move the mouse over the folder pane and the plus signs appear on folders that have subfolders. When I move my mouse to the file pane, the plus signs disappear from the folders pane.
Crap, now it's not doing it, which is what I want, but I hope this change is permanent. I wonder if there are different types of Explorer windows that show files?
EDIT: When my c: drive is chosen in the folders pane, the plus signs are on all the time. But when an external HD is selected in the folders pane, the plus signs flash on and off. How do I stop this flashing for all types of drives?